Rare gas halide excimer lasers are high-pressure gas lasers which utilize halogens such as fluorene and chlorine in small concentrations.
Halogens are chemically reactive gases and the energy, which results from the laser discharge circuit, effects undesirable reactions resulting in the formation of impurities. These impurities absorb the laser light generated within the laser cavity, causing loss of laser emission. These impurities can also cause degradation of the discharge itself, reducing laser efficiency and component lifetime.
Other impurities can be introduced by opening the laser to ambient air whereby water vapor is absorbed into the laser vessel and other internal components, impurities inherent in the laser supply gases or the gas handling system and from reactions based on the materials of construction of the laser itself.
Cyrogenic liquid nitrogen trapping is a widely accepted method of purification of the gases used in excimer lasers. The excimer laser gas is pumped from the laser head into a cold trap. Within the cold trap, the laser gas is purified by freezing the impurities and then filtering the frozen impurities. The temperature of the cold trap must be held at a temperature low enough to freeze out the volatile impurities but not so low as to tend to liquify the laser gases. Gases, such as argon fluoride which lases at 193 nm and krypton fluoride which lases at 248 nm, benefit greatly from the removal of impurities. These laser gases require cold trap temperatures which are above the liquid nitrogen temperature (at atmospheric pressure) to avoid condensing out of the rare gases. Other laser gas mixtures, such as xenon chloride which lases at 308 nm, require temperatures well above the 77K temperature of liquid nitrogen to avoid liquifying both the rare gas and the halogen.
Prior art devices use atmospheric liquid nitrogen to cool the cold trap device. These cold traps have been found to work well with argon fluoride because the resulting temperatures of the cold trap maintain an argon vapor pressure adequate for lasing. For use with other lasing gases such as krypton fluoride or xenon chloride, a thermal impedence is placed between the liquid nitrogen and the cold trap. Further, an electronic temperature controller is used to control a heating element to stabilize the temperature at an elevated level.
These devices require frequent replenishment of the liquid nitrogen typically from a large storage Dewar.
The large storate Dewar is usually pressured to 20 to 230 psi. When the liquid nitrogen from the storatge Dewar is transferred to the cold trap, losses due to flashing can be considerable. Further, the frequent handling and filling requirements are quite inconvenient, inefficient and unreliable for industrial use. A further drawback to the cold trap is the low thermal insulating efficiency of the liquid nitrogen reservoir in the cold trap.
The art has also suggested a thermal mechanical refrigerator to cool the cold trap. This method has a high monitary cost associated with the refrigeration unit and complex designs to achieve insulation.
The present invention overcomes the drawbacks of the prior art devices by using a pressurized Dewar together with a regenerative heat exchange system. The storage portion of the Dewar is pressurized whereby the temperature of the liquid nitrogen can be controlled. The large liquid nitrogen volume of the device limits the amount of fill intervals thereby lowering flash loss.
Thus, the advantages of my invention are lengthening the time that a laser system can run unattended by reducing the number of liquid nitrogen fills from many times a day to one to two, or less than one time per week. The need for a large storage vessel near the laser system is not necessary and there is a resultant reduction in the lost volume of liquid nitrogen due to flash loss. Further, the thermodynamic efficiency of the cold trap of the invention is significantly enhanced by controlling the temperature of the cold trap by pressurizing the liquid nitrogen rather than using electric heating elements.